This invention relates generally to an optical disc player in which a reading light beam is caused to impinge on an optical disc having information recorded thereon and then the reading light beam coming from the optical disc is detected to reproduce the information, and more particularly, is directed to an improved optical disc player in which a tracking error signal used for controlling a reading light beam to impinge correctly on an optical disc can be obtained through an arrangement of optical components relatively simplified in configuration and tracking control for the reading light beam can be accurately carried out in accordance with the tracking error signal thus obtained.
For an optical disc player in which a light beam is utilized for reproducing information from an optical disc having thereon a record track which is formed with an alignment of geometric variations such as a plurality of pits provided in response to the information, it is required to perform focus control for maintaining correct focus of the light beam caused to impinge on the optical disc and tracking control for maintaining the light beam in correct tracking relation to the record track on the optical disc being traced thereby. In order to carry out such focus control and tracking control, the optical disc player is operative to detect defocus of the light beam at the record track on the optical disc and produce a first error signal representing the detected defocusing of the light beam and also to detect positional deviation of the light beam from the center of the record track on the optical disc and produce a second error signal representing the detected positional deviation of the light beam. Usually, the first error signal is obtained as a focus error signal and the second error signal is obtained as a tracking error signal in the optical disc player.
There has been proposed a tracking error signal producing device having a relatively simple arrangement of optical components as shown in FIG. 1. In the device of FIG. 1, a laser light beam emitted from a laser light source 1 is collimated by a collimating lens 2 and then enters through a beam splitter 3 and a quarter-wave plate 4 into an object lens 5 to pass through the same to be caused to impinge thereby on a disc 6 as a reading light beam. The disc 6 has a spiral record track formed with an alignment of pits each having the depth corresponding to, for example, a quarter of the wavelength of the laser light beam from the laser light source 1 and provided in response to information recorded thereby, and is rotated so as to keep the tangential velocity of the spiral record track relative to the laser light beam caused to impinge thereon constant at a predetermined value. The laser light beam reflected from the disc 6, which has been modulated in intensity in accordance with the spiral record track on the disc 6, again enters into the object lens 5 to pass through the same and then enters through the quarter-wave plate 4 into the beam splitter 3. Due to the effect of the quarter-wave plate 4, the reflected laser light beam entering into the beam splitter 3 is linearly polarized in the direction perpendicular to the direction in which the laser light beam leaving from the beam splitter 3 for the disc 6 is linearly polarized. Therefore the reflected laser light beam is reflected at the beam splitter 3 and lead to a photodetector 7. Thus, the reflected laser light beam which has been modulated in intensity in accordance with the spiral record track on the disc 6, that is, the reflected reading light beam is detected by light detecting elements forming the photodetector 7 and electric signals are produced by the light detecting elements in response to the variations in intensity of the reflected reading light beam. These electric signals obtained from the photodetector 7 are supplied to an error signal producing circuit and a tracking error signal which is to be used for controlling, for example, the position of the object lens 5 to perform the tracking control is produced by the error signal producing circuit.
The above mentioned photodetector 7 comprises, for example, four light detecting elements D.sub.1, D.sub.2, D.sub.3 and D.sub.4 as shown in FIG. 2 and the reflected reading light beam from the beam splitter 3 forms its beam spot on the light detecting elements D.sub.1 -D.sub.4, as shown by a broken line in FIG. 2. The light detecting elements D.sub.1 -D.sub.4 produce the respective output signals each corresponding to a portion of the beam spot formed on each of the light detecting elements D.sub.1 -D.sub.4 at respective output terminals d.sub.1 -d.sub.2.
Now, the tracking control will be considered hereinafter. The spiral record track on the disc 6 is formed with the arrangement of the pits each having the depth corresponding to a quarter of the wavelength of the reading light beam caused to impinge thereon and the reading light beam irradiating the spiral record track is diffracted by the pits to be reflected thereat. Accordingly, the reflected reading light beam returning through the object lens 5 and attaining to the photodetector 7 to form the beam spot on the light detecting elements D.sub.1 -D.sub.2 forms a diffraction pattern varying in response to the positional relation between each pit on the disc 6 and the beam spot on the disc 6 formed by the reading light beam irradiating the pit. FIGS. 3A, 3B and 3C shown such diffraction pattern and positional relation obtained in several different situations. In each of FIGS. 3A, 3B and 3C, m indicates the positional relation between the pit p and the beam spot 1 of the reading light beam, and n indicates the diffraction pattern (a shaded portion) formed at the exit pupil plane of the object lens 5 by the reflected reading light beam in consequence of the positional relation indicated by m. Four divided portions D.sub.1 ', D.sub.2 ', D.sub.3 ' and D.sub.4 ' in n shown areas which are to be light-detected by the light detecting elements D.sub.1, D.sub.2, D.sub.3 and D.sub.4, respectively. The pit p moves in relation to the beam spot 1 so that the situation indicated by t.sub.a is changed into the situation indicated by t.sub.b. In the case of FIG. 3A, the beam spot 1 is deviated on the right side from the center of the pit p. In the case of FIG. 3B, the beam spot 1 is located at the center of the pit p, that is, the reading light beam is maintained in correct tracking relation to the spiral record track on the disc 6. Further, in the case of FIG. 3C, the beam spot 1 is deviated on the left side from the center of the pit p.
From FIGS. 3A, 3B and 3C, it is understood that the diffraction pattern by which the divided portions D.sub.1 ', D.sub.2 ', D.sub.3 ' and D.sub.4 ' are supplied with the same light amount, respectively, is obtained when the beam spot 1 is located at the center of the pit p, that is, the reading light beam is maintained in correct tracking relation to the spiral record track, and such diffraction pattern that the light amount supplied to the divided portions D.sub.1 ', D.sub.2 ', D.sub.3 ' and D.sub.4 ' is made asymmetric when the beam spot 1 is deviated on the right or left side from the center of the pit p and the manner of asymmetry of the light amount in the case of the deviation on the right side is contrary to the manner of asymmetry of the light amount in the case of the deviation on the left side. Consequently, it is also understood that a signal varying in response to the positional relation between the beam spot 1 and the pit p, that is, an tracking error signal can be obtained by processing in an appropriate error signal producing circuit the outputs of the light detecting elements D.sub.1, D.sub.2, D.sub.3 and D.sub.4 which detect the light amount supplied to the divided portions D.sub.1 ', D.sub.2 ', D.sub.3 ' and D.sub.4 ', respectively. The tracking error signal thus obtained is to be used for driving, for example, the object lens 5 to move the position thereof in order to maintain the situation in which the beam spot 1 is located at the center of the pit p, as shown in FIG. 3B.
FIG. 4 shows an example of the error signal producing circuit for making the tracking error signal from the outputs of the above mentioned light detecting elements D.sub.1, D.sub.2, D.sub.3 and D.sub.4. In this circuit, the outputs of the light detecting elements D.sub.1 and D.sub.4 are added to each other in an adding circuit 11, and the outputs of the light detecting elements D.sub.2 and D.sub.3 are added to each other in an adding circuit 12. Then, the subtraction between the outputs of the adding circuits 11 and 12 is performed in a subtracting circuit 13, and further the outputs of the adding circuits 11 and 12 are added to each other in an adding circuit 14.
When the beam spot formed by the reading light beam caused to impinge on the disc 6 moves to traverse to spiral record track formed with the arrangement of the pits from the right to the left, for example, a subtracted signal S.sub.1 as shown in FIG. 5A is obtained an the output end of the subtracting circuit 13 and an added signal S.sub.2 as shown in FIG. 5B is obtained at an output terminal of the adding circuit 14. The subtracted signal S.sub.1 is such a signal as to vary whenever the beam spot formed by the reading light beam passes through each pit and reside in the frequency band of the recorded information signal, and has positional information representing the position of the beam spot formed by the reading light beam in relation to the spiral record track, while the added signal S.sub.2 is a reproduced information signal which will be matured into reproduced information. The added signal S.sub.2 from the adding circuit 14 is supplied to a rising pulse generating circuit 15 so that a pulse S.sub.3 as shown in FIG. 5C is obtained in response to each rising zero crossover point of the added signal S.sub.2 at the output terminal thereof and also supplied to a falling pulse generating circuit 16 so that a pulse S.sub.4 as shown in FIG. 5D is obtained in response to each falling zero crossover point of the added signal S.sub.2 at the output terminal thereof. The subtracted signal S.sub.1 from the subtracting circuit 13 is supplied to sampling-and-hold circuits 17 and 18. In the sampling-and-hold circuit 17, the level of the subtracted signal S.sub.1 is sampled by the pulse S.sub.3 and the sampled level is held, so that an output signal S.sub.5 as shown in FIG. 5E is derived therefrom, and in the sampling-and-hold circuit 18, the level of the subtracted signal S.sub.1 is sampled by the pulse S.sub.4 and the sampled level is held, so that an output signal S.sub.6 as shown in FIG. 5F is derived therefrom. Each of the output signals S.sub.5 and S.sub.6 varies in its polarity to positive from negative or to negative from positive when the position of the beam spot formed by the reading light beam moves to the left from the right in relation to the spiral record track and has the level corresponding to the deviation of the beam spot formed by the reading light beam from the center of the spiral record track. Therefore, each of the output signals S.sub.5 and S.sub.6 can be used as the tracking error signal. The output signals S.sub.5 and S.sub.6 are supplied to a differential circuit 19 which performs the subtraction between the output signals S.sub.5 and S.sub.6 to produce a tracking error signal S.sub.7 at an output terminal 20. This tracking error signal S.sub.7 is supplied to, for example, a driving circuit provided for driving the object lens 5.
However, in general, a tracking error signal obtained in such a manner as mentioned above is influenced by inclination of a disc surface having a spiral record track thereon in regard to a plane perpendicular to the optical axis of a reading light beam caused to impinge on the disc surface (hereinafter referred to as disc inclination) to vary its level in response to the degree of the disc inclination. For example, even if the amount of deviation of the beam spot formed by the reading light beam from the center of the spiral record track, that is, the amount of tracking error of the reading light beam is constant, the level of the tracking error signal is reduced in proportion as the disc inclination increases. In addition to this, the influence of the disc inclination upon the tracking error signal depends on the size of a pit, which is provided on the disc surface for forming the spiral record track, in the direction along the spiral record track (hereinafter referred to as the pit length), and the manner of variations in the level of the tracking error signal caused by the disc inclination is varied in response to the pit length. For example, in case that the reading light beam generated through the arrangement of optical components shown in FIG. 1 is caused to impinge on a disc having a spiral record track formed with a plurality of pits each having the same pit length and aligned with a space equal to the pit length between each adjacent two of them, and a tracking error signal is produced by the error signal producing circuit shown in FIG. 4 in response to the reflected reading light beam from the disc, the relation between the level L of the subtracted signal S.sub.1 derived from the subtracting circuit 13 in response to a constant amount of tracking error and disc inclination K is obtained as shown in FIG. 6 with the parameter of the spatial frequency F of an alignment of pits. As understood from FIG. 6, reduction of the level L of the subtracted signal S.sub.1 caused in proportion to increase of the disc inclination K is made more and more steep in proportion as the spatial frequency F decreases, as shown by lines .alpha., .beta. and .gamma. obtained in response to the spatial frequencies of 500 mm.sup.-1, 333 mm.sup.-1 and 250 mm.sup.-1, respectively. This means that the variations in the level L of the subtracted signal S.sub.1 obtained in response to a constant amount of tracking error caused by the disc inclination are more and more increased in proportion as the spatial frequency F decreases, that is, in proportion as the pit length increases. In practice, since the spiral record track on a disc is formed with an alignment of pits having various pit length in a predetermined range, the manner of variations in the level L of the subtracted signal S.sub.1 caused by the disc inclination K is varied in response to the pit length and accordingly the tracking error signal made from such subtracted signal S.sub.1 has undesirable level variations depending on the pit length. This results in that a proper tracking error signal responding correctly to the tracking error is not obtained and consequently tracking control is carried out inaccurately.